1. Field
The disclosed concept pertains generally to electromagnetic actuators and, more particularly, to solenoids.
2. Background Information
Electromagnetic actuators, such as solenoids, are used for many different applications. A solenoid provides an electromagnetic force in response to electrical power applied to its terminals, Solenoids can include an air core or an iron core. In iron core solenoids, a magnetic frame cooperates with magnetic flux produced by a coil in order to provide a closed, low reluctance magnetic path for the magnetic flux. The coil is wound on a bobbin and mounted inside the magnetic frame. Solenoids also include a moving core or armature and a fixed core or pole. The magnetic flux completes a path from the pole through a magnetic gap to the armature to the magnetic frame and back to the pole. In this complete travel of the magnetic flux, there is some amount of magnetic flux (i.e., a leakage flux) which does not reach the armature. This leakage flux is wasted and cannot contribute toward producing a magnetic force. Therefore, for effective and efficient use of solenoids, the amount of leakage flux should be minimized, in order that the magnetic force can be maximized.
Referring to FIG. 1, a solenoid 2 includes a magnetic frame 4, a hold coil 6, a pick up coil 8, a bobbin 10, a fixed core (pole) 12, a moving core (armature) 14, a return spring 16 and a plunger 18. Solenoids, such as the solenoid 2, have two extreme positions including a first position (or pick up state) when the armature 14 and the pole 12 are separated by a maximum possible gap (or magnetic gap 20 of FIGS. 1 and 2), and a second position (or holding state) when the armature 14 and the pole 12 are proximate (e.g., almost touching) each other (as shown in phantom line drawing in FIG. 1). The solenoid pick up state occurs when an electrical power supply (not shown) is not provided to the coil terminals (not shown) for the hold coil 6 and the pick up coil 8. After the electrical power supply is provided to the coil terminals in the pick up state, the coils 6,8 carry some amount of current depending upon the solenoid state, the coil impedance and the number of coil winding turns. The number of turns (N) and the current (I) carried by the coils 6,8 determine the total NI across the coil terminals. The amount of NI across the coils 6,8 and the magnetic gap 20 determine the value of the magnetic flux in the solenoid 2.
The pick up coil 8 and the hold coil 6 can be wound either in series or in parallel. Normally, there is no electrical connection between the coils 6,8 in the solenoid 2, and they are electrically connected in series or in parallel through an “economizer” circuit (not shown). A suitable “economizer” or “cut-throat” circuit (not shown) can be employed to de-energize the pick up coil 8 in order to conserve power and minimize heating in the solenoid 2 in the holding state. The economizer circuit can be implemented by a timing circuit (not shown) which pulses the pick up coil 8 only for a predetermined period of time, proportional to the nominal armature operating duration. This is achieved by using a dual coil arrangement in which there is a suitable relatively low resistance circuit or coil and a suitable relatively high resistance circuit or coil in series with the former coil. Initially, the economizer circuit allows current to flow through the low resistance circuit, but after a suitable time period, the economizer circuit turns off the low resistance path. This approach reduces the amount of power consumed during static states (e.g., relatively long periods of being energized).
The example winding approach employed in FIG. 1 is such that the pick up coil 8 is wound first across about the entire height (with respect to FIG. 1) of the bobbin 10 and then the hold coil 6 is wound over about the entire height (with respect to FIG. 1) of the pick up coil 8.
There is room for improvement in solenoids.